This invention pertains generally to missile flight control systems and particularly to those in which the missile is a body of revolution with four movable control surfaces, commonly referred to as "control fins," in a cruciform array.
It is well known in the art that a missile of the type herein contemplated for guided atmospheric flight may be required to maneuver in any lateral direction during an interception of a target and that any required maneuver may be effected by appropriate aerodynamic lift forces determined by a flight control system. In order to permit a maneuver in any lateral direction regardless of angle of attack, such a missile has at least two axial planes of symmetry forming a cruciform cross-section on the arms of which four identical movable control surfaces (and sometimes four identical fixed surfaces) are disposed. The flight control system typically consists of a pitch autopilot and a yaw autopilot for controlling lateral acceleration of the missile in two mutually orthogonal planes and, additionally, a roll autopilot. The roll autopilot is required to maintain a nominally constant roll attitude so that the other two autopilots can function normally without compensating for rapid rolling and so that the missile radar seeker (if such a seeker is used) can maintain the proper orientation with respect to the polarization of external radar signal. In many missiles, each autopilot has a rate gyroscope for damping feedback, and in the case of pitch and yaw, an accelerometer for the measurement and feedback of lateral acceleration in, respectively, the pitch and yaw planes. The autopilots then generate commands for a control actuation system which in turn moves the four control fins as required to effect any desired maneuver.
The control actuation system includes conventional actuators to turn the control fins relative to the body of the missile and, in some missiles, other apparatus such as servoamplifiers and potentiometers to control fin angle. The primary purpose of the control fins is to develop aerodynamic moments on the missile for flight control, although a secondary purpose may be to develop lift for lateral acceleration as in a canard-controlled missile. In the case when the missile guidance system calls for a constant pitch acceleration, the pitch autopilot changes the deflection of the appropriate control surfaces; at equilibrium, however, fin deflections are constant and a constant angle of attack is maintained, with constant lateral acceleration in the pitch plane.
A practical interceptor missile must rely mainly on aerodynamic lift of its body at an angle of attack to develop the necessary large lateral accelerations for homing on a target (which ordinarily is moving and which may be taking evasive action). In general, such aerodynamic lift is developed by controlling the angle of attack (meaning the angle between the longitudinal centerline and the velocity vector of the missile) with control moments from the control fins. It is evident larger air pressures exist on the windward portions of the missile than on leeward portions and that concomitant differences in air pressure exist on the different control fins. For a high dynamic pressure of the free-stream, e.g. at high Mach number and low altitude, only a small angle of attack is necessary for a moderate lateral acceleration. In this case the four control surfaces (and four fixed surfaces, if provided) are about equally effective, and the three autopilots function independently without significant coupling effects.
For high altitude flight with a low free-stream dynamic pressure, a large angle of attack, e.g. over 20.degree., may be required to develop sufficient aerodynamic lift for homing. In this case the windward control surfaces are subjected to appreciably higher air pressures than the leeward surfaces. Moreover, the nose and wiring humps (if any) cause air vortices which change the air pressure distribution over the various control surfaces. Hence, the effectiveness, i.e. the lift force per degree of deflection, is higher for a windward control surface than for the diametrically opposite leeward control surface. Each of the three conventional autopilots deflects one or two diametral pairs of the control surfaces through equal deflections. The final result in such a situation is that the pitch or yaw fin deflections can induce unwanted roll moments, while the roll fin deflections can induce unwanted pitch or yaw moments. Such induced unwanted moments may be termed "aerodynamic cross-coupling."
The problem of aerodynamic cross-coupling is not restricted to a missile with a simple body-tail configuration. The problem is complicated, with a winged missile, by the vortices formed by the wings, or strakes. Such vortices impinge on the control surfaces and change the distribution of local air pressures on such surfaces. The problem is also complicated with a canard-controlled missile where movable forward surfaces cause a changing downwash impinging on the aft fixed surfaces to induce unwanted moments.
Such aerodynamic cross-coupling causes unwanted rolling of the missile, interferes with proper guidance and may even cause a loss of stability in the flight control system.
In order to overcome the problems of aerodynamic cross-coupling, various missile configurations have been proposed. Thus, for example, a "ring-tailed" species of missile which has a bracelet-shaped airfoil supported by fixed thin struts from the body of the missile with an annular space has been devised. The airfoil provides the desired lift force with a minimum of an unwanted roll moment. Unfortunately, however, such a missile is expensive and the airfoil cannot be deflected to allow complete control of missile direction. Another type of missile utilizes a spinning tail section supported on a roll bearing; such an arrangement is not susceptible to induced roll moments, but cannot counteract roll moments due to other reasons.
At the present state of development of the art, most guided missiles utilize a cruciform configuration for the control surfaces or fins. A newly considered "interdigitated" missile of such configuration has four movable tail fins and four fixed strakes alternately and evenly disposed around the missile. During an intercept the missile is rolled so that required lateral maneuvers take place in the most favorable plane relative to the control fins. While the missile may be so rolled during the initial and midcourse phases of an intercept, in the rapidly changing terminal phase of homing, the requisite control of roll attitude is almost impossible to achieve. Thus, although cross-feeds between the roll and yaw autopilots are provided, the flight-control-system stability of such a missile is still somewhat marginal at high angles of attack. More recently, it has been proposed to utilize an on-board digital computer in a cruciform missile to develop fin commands which will minimize adverse aerodynamic cross-coupling, but this technique will require appreciable data storage and computation.